1. Field
The present disclosure relates generally to the implantable portion of implantable cochlear stimulation (or “ICS”) systems.
2. Description of the Related Art
ICS systems are used to help the profoundly deaf perceive a sensation of sound by directly exciting the intact auditory nerve with controlled impulses of electrical current. Ambient sound pressure waves are picked up by an externally worn microphone and converted to electrical signals. The electrical signals, in turn, are processed by a sound processor, converted to a pulse sequence having varying pulse widths, rates and/or amplitudes, and transmitted to an implanted receiver circuit of the ICS system. The implanted receiver circuit is connected to an implantable electrode array that has been inserted into the cochlea of the inner ear, and electrical stimulation current is applied to varying electrode combinations to create a perception of sound. The electrode array may, alternatively, be directly inserted into the cochlear nerve without residing in the cochlea. A representative ICS system is disclosed in U.S. Pat. No. 5,824,022, which is entitled “Cochlear Stimulation System Employing Behind-The-Ear Sound processor With Remote Control” and incorporated herein by reference in its entirety. Examples of commercially available ICS sound processors include, but are not limited to, the Harmony™ BTE sound processor, the Naida™ CI Q Series sound processor and the Neptune™ body worn sound processor, which are available from Advanced Bionics.
As alluded to above, some ICS systems include an implantable cochlear stimulator (or “cochlear implant”), a sound processor unit (e.g., a body worn processor or behind-the-ear processor), and a microphone that is part of, or is in communication with, the sound processor unit. The cochlear implant communicates with the sound processor unit and, some ICS systems include a headpiece that is in communication with both the sound processor unit and the cochlear implant. The headpiece communicates with the cochlear implant by way of a transmitter (e.g., an antenna) on the headpiece and a receiver (e.g., an antenna) on the implant. Optimum communication is achieved when the transmitter and the receiver are aligned with one another. To that end, the headpiece and the cochlear implant may include respective positioning magnets that are attracted to one another, and that maintain the position of the headpiece transmitter over the implant receiver. The implant magnet may, for example, be located within a pocket in the cochlear implant housing. The skin and subcutaneous tissue that separates the headpiece magnet and implant magnet is sometimes referred to as the “skin flap,” which is frequently 3 mm to 10 mm thick.
The magnitude of the retention force between the headpiece magnet and implant magnet is an important aspect of an ICS system. If the force is too low, the headpiece will not remain in place on the head during typical activities. If, on the other hand, the force is too high, the pressure on the skin flap can result is discomfort and tissue necrosis. The magnitude of the retention force is dictated by the strength of the magnets and the distance between the magnets, which is a function of the thickness of the skin flap. The strength of the headpiece magnet is frequently selected during the post-implantation headpiece fitting processes.
The present inventors have determined that conventional cochlear implants are susceptible to improvement. For example, the magnets in many conventional cochlear implants are disk-shaped and have north and south magnetic dipoles that are aligned in the axial direction of the disk. Such magnets are not compatible with magnetic resonance imaging (“MRI”) systems. In particular, the cochlear implant 10 illustrated in FIG. 1 includes, among other things, a housing 12 and a disk-shaped solid block magnet 14. The implant magnet produces a magnetic field M in a direction that is perpendicular to the patient's skin and parallel to the axis A, and this magnetic field direction is not aligned with, and may be perpendicular to (as shown), the direction of the MRI magnetic field B. The misalignment of the interacting magnetic fields M and B is problematic for a number of reasons. The dominant MRI magnetic field B (typically 1.5 Tesla or more) may demagnetize the implant magnet 14 or generate a significant amount of torque T on the implant magnet 14. The torque T may dislodge the implant magnet 14 from the pocket within the housing 12, reverse the magnet 14 and/or dislocate the cochlear implant 10, all of which may also induce tissue damage. One proposed solution involves surgically removing the implant magnet 14 prior to the MRI procedure and then surgically replacing the implant magnet thereafter.
One proposed solution involves the use of freely rotatable ball magnets that create a magnetic field which can rotate, from the aforementioned direction that is perpendicular to the patient's skin, to a direction that is aligned with the direction of the MRI magnetic field B. To that end, and referring to FIG. 2, one proposed implantable magnet apparatus 20 includes a plurality of freely rotatable ball magnets 22 within a case 24. When the magnet apparatus 20 is in very close proximity to an external magnet 26, the ball magnets 22 will align with the external magnet 26 in the manner shown, with the N-S direction of the ball magnets being the same as that of the external magnet. When the external magnet 26 is removed (FIG. 3), the ball magnets 22 will align with one another. The ball magnets 22 will then rotate as necessary in response to the application of the MRI magnetic field, thereby minimizing the torque T, because the MRI magnetic field is far stronger than the attraction between the ball magnets. Turning to FIG. 4, the present inventors have determined that the use of freely rotatable ball magnets 22 is less than optimal because the distance between implanted ball magnets (located within a cochlear implant 28) and the external magnet 26 (located within an external headpiece 30) is so great that the magnetic attraction between the ball magnets is greater than the magnetic attraction between the ball magnets and the external magnet. The N-S direction of the ball magnets 22 is perpendicular to the N-S direction of the external magnet 26. The increased distance, as compared to the distance illustrated in FIG. 3, is a product of, for example, the presence of the implant and headpiece housings and the thickness of the skin flap. The weak magnetic attraction resulting from the misalignment of the magnetic fields prevents the headpiece from properly mounting to the patient's head. One possible solution is to simply increase the size of the external magnet, thereby increasing the strength of the associated magnetic field to the point at which the ball magnets 22 in a cochlear implant will rotate into the orientation illustrated in FIG. 2. The present inventors have determined, however, that the associated increase in the size and weight of the headpiece is undesirable.
Another proposed solution is illustrated in FIG. 5. Here, the cochlear implant 32 includes a magnet apparatus with a diametrically magnetized disk-shaped magnet 34 that is rotatable relative to the remainder of the implant about an axis A, and that has a N-S orientation which is perpendicular to the axis A. The external headpiece 36 includes a diametrically magnetized disk-shaped magnet 38 that is not rotatable relative to the remainder of the headpiece. The implanted magnet 34 is able to rotate about the axis A into alignment with the external magnet 38, and is also able to rotate about the axis A into alignment with an MRI magnetic that is perpendicular to the axis A (and parallel to the N-S direction). The present inventors have determined that the use of the diametrically magnetized disk-shaped magnet 34 is less than optimal because a dominant magnetic field, such as the MRI magnetic field B, that is misaligned by at least 30° (FIG. 6) or more (FIG. 6A) from the N-S direction of the magnet may demagnetize the magnet or generate an amount of torque T1 (FIG. 6) or a larger torque T1a (FIG. 6A) on the magnet that is sufficient to dislodge or reverse the magnet apparatus and/or dislocate the associated cochlear implant and/or cause excessive discomfort to the patient.